Dielectric (or insulating) fluids used in electrical distribution and power equipment—including transformers, switching gear and electric cables—perform two important functions. These fluids act as an electrical insulating medium, i.e., exhibit dielectric strength, and they transport generated heat away from the equipment, i.e., act as a cooling medium. When used in a transformer, for example, dielectric fluids transport heat from the windings and core of the transformer or connected circuits to cooling surfaces. Apart from possessing dielectric strength and cooling capacity, an ideal dielectric fluid for electrical equipment also exhibits little or no detrimental impact on the environment, is compatible with materials used to construct the equipment, and is relatively nonflammable.
For more than a century, mineral oils derived from crude petroleum were used extensively as insulating and cooling liquids in electrical equipment. Though such oils possess a satisfactory dielectric strength and are compatible with equipment materials, they are not considered nonflammable, and, because they are petroleum-based, they are considered to carry with them an environmental cost. In the middle part of this century, as safety standards became more demanding for many indoor and vault equipment installations, mineral oils were replaced to a large extent by nonflammable liquids such as askarel (polychlorinated biphenyl, or PCB) fluids. Beginning in the 1930s, for example, PCBs—which generally are considered nonflammable—were used extensively to replace mineral oils in fire sensitive locations as insulating fluids in electrical equipment.
PCBs eventually were recognized for their environmental hazards, and as a result the production and sale of PCBs as well as their use in new equipment was banned. For existing PCB-filled equipment, stringent regulations now require removal of PCB fluids at certain installations and, for all other installations, place stringent restrictions on the use of PCB-filled equipment. Spill reporting, clean-up, and disposal of PCB-filled equipment also now require compliance with very strict EPA regulations.
Because of the disadvantages and shortcomings of PCB-based fluids and because of the increasing sensitivity to the potential adverse environmental impact of mineral oils and available alternatives, there have been and continue to be numerous efforts undertaken to develop relatively inexpensive, environmentally safe, and nonflammable dielectric fluids. To date, these efforts have not been completely successful.
There are a number of specific functional properties characteristic of dielectric oils. An oils dielectric breakdown, or dielectric strength, for example, provides an indication of its ability to resist electrical breakdown and is measured as the minimum voltage required to cause arcing between two electrodes at a specified gap submerged in the oil. The impulse dielectric breakdown voltage provides an indication of an oils ability to resist electrical breakdown under transient voltage stresses such as lightning and power surges. The dissipation factor of an oil is a measure of the dielectric losses in the oil; a low dissipation factor indicates low dielectric loss and a low concentration of soluble, polar contaminants. The gassing tendency of an oil measures the oils tendency to evolve or absorb gas under conditions where partial discharge is present.
Because one function of a dielectric fluid is to carry and dissipate heat, factors that significantly affect the relative ability of the fluid to function as a dielectric coolant include viscosity, specific heat, thermal conductivity, and the coefficient of expansion. The values of these properties, particularly in the range of operating temperatures for the equipment at full rating, must be weighed in the selection of suitable dielectric fluids for specific applications.
In addition to the foregoing properties that affect heat transfer, a dielectric fluid, to be useful in commercial applications, should have a relatively high dielectric strength, low dissipation factor, a dielectric constant that is compatible with the solid dielectric, a low gassing tendency, and it must be compatible with the electrical equipment materials to which it is exposed.
Current codes and standards require further that any dielectric fluid intended for use as a coolant not be classified as Flammable, but rather as a Class IIIB Combustible liquid. Specific safety requirements, however, vary with the application to which the electric equipment containing the fluid is used. Such applications include, for example, indoor and rooftop installations, vault applications, and installations adjacent to building structures. According to the degree of hazard attendant to these varied applications, one or more additional safeguards may be required. One recognized safeguard is the substitution of conventional mineral oils with less-flammable and/or nonflammable liquids. Less-flammable liquids are considered to be those having an open-cup fire point equal to or greater than 300° C.
Several dielectric fluids are known and used in electrical equipment. Due, however, to an increasing awareness and sensitivity toward environmental concerns, it has become increasingly desirable to provide a dielectric fluid that: (1) poses minimal environmental hazards; (2) degrades quickly and easily so that spills do not contaminate the soil or the water table for any significant period of time; and (3) does not interfere in any significant way with natural biodegradation processes. It also is becoming more desirable to replace non-renewable resources with renewable resources, particularly given the undesirability of dependence on petroleum-derived products, and there generally is increased demand by the industrial and retail markets for all-natural products. This is due, at least in part, from the attention paid to the long-term effects of materials and their degradation by-products.
In prior, related co-pending application Ser. No. 08/728,261, now U.S. Pat. No. 6,037,537—which is incorporated in its entirety by reference—we described a class of insulating dielectric fluids comprising vegetable oil materials. These compositions, useful in electrical distribution and power equipment, utilize low maintenance vegetable oil-based dielectric coolants that meet or exceed applicable safety and performance standards and that are free of substantial environmental hazards.